Since nonwoven fabrics have excellent breathability and softness, they have been widely used for surface sheets and back sheets of absorbent articles such as disposable diapers and sanitary napkins, which are required to have absorbing properties for quickly transferring liquids excreted or discharged such as blood and urine to absorbers, and surface properties for providing wearer's and handler's skins contacting with the surface with softness and low irritation.
As a method of improving the skin touch properties and wet backing properties concerning surface sheets, Patent document 1 discloses a method of laminating a high shrinkable fiber sheet with a low shrinkable or non-shrinkable nonwoven fabric and heat treating the laminate thereby shrinking the high shrinkable fiber sheet to form wrinkles on the surface. Patent document 2 discloses a method of laminating a bulky nonwoven fabric provided with bulkiness by hot air treatment with a shrink potential-having fiber nonwoven fabric capable of being shrunk by heat treatment.
Furthermore, as a method for improving absorption and transferring rate of liquids such as urine discharged, Patent document 3 discloses a method of preparing a nonwoven fabric having a complex phase structure with a low water permeating part that has a bulky layered structure of a hydrophobic fiber layer and a hydrophilic fiber layer, and a water permeating part in which a hydrophobic fiber and a hydrophilic fiber are mingled. Patent document 4 discloses a method of preparing a nonwoven fabric by mixing a hydrophilic fiber and a water repellent fiber.
The heat treatment method for providing a nonwoven fabric with bulkiness, however, has a complicated process. Furthermore, the heat treatment does not thin fibers forming the nonwoven fabric, and possibly involve heat shrinkage increasing the diameter of fibers; anyhow, the heat treatment may lead to reduced softness and touch. Further, the nonwoven fabric obtainable by mixing the hydrophilic fiber and the water repellent fiber is insufficient in liquid dispersibility, failing to provide a surface sheet simultaneously having softness and liquid dispersibility.
Patent document 5 discloses adding a nonionic surface-active agent such as polyoxyethylene alkylether in order to hydrophilize a nonwoven fabric such as a spunbonded nonwoven fabric composed of a propylene polymer. It is found, however, that by the addition of the surface-active agent, the initial hydrophilicity is improved but the long-lasting hydrophilicity is inferior. The term “initial hydrophilicity”, representing such a hydrophilicity as used in a general broad sense, is optionally used in the present specification in order to be distinguished from the long-lasting hydrophilicity, which represent a hydrophilicity observed after the exposure to environment of a temperature somewhat higher than room temperature, i.e., about 40° C., for a certain period of time. In the present specification, this meaning is applied to the term. “long-lasting hydrophilicity” hereinafter.
On the other hand, the back sheet has been required to widely disperse liquids excreted or discharged such as blood, urine and sweat, and evaporate the absorbed liquids for a short period of time. As a method to realize improvement in this regard, Patent document 6, for example, proposes a method for preparing a diaper by combining a hydrophobic sheet and a hydrophilic sheet.
The method for combining the sheets, however, has a complicate process. Furthermore, since only apart of the diaper absorbs liquid, the liquid may remain within the diaper.